Identification of chick corneal keratan sulfate proteoglycan precursor protein in whole corneas and in cultured corneal fibroblasts

Molecular cues for immune cells from small leucine-rich repeat proteoglycans in their extracellular matrix-associated and free forms

In this review we highlight emerging immune regulatory functions of lumican, keratocan, fibromodulin, biglycan and decorin, which are members of the small leucine-rich proteoglycans (SLRP) of the extracellular matrix (ECM). These SLRPs have been studied extensively as collagen-fibril regulatory structural components of the skin, cornea, bone and cartilage in homeostasis. However, SLRPs released from a remodeling ECM, or synthesized by activated fibroblasts and immune cells contribute to an ECM-free pool in tissues and circulation, that may have a significant, but poorly understood foot print in inflammation and disease. Their molecular interactions and the signaling networks they influence also require investigations. Here we present studies on the leucine-rich repeat (LRR) motifs of SLRP core proteins, their evolutionary and functional relationships with other LRR pathogen recognition receptors, such as the toll-like receptors (TLRs) to bring some molecular clarity in the immune regulatory functions of SLRPs. We discuss molecular interactions of fragments and intact SLRPs, and how some of these interactions are likely modulated by glycosaminoglycan side chains. We integrate findings on molecular interactions of these SLRPs together with what is known about their presence in circulation and lymph nodes (LN), which are important sites of immune cell regulation. Recent bulk and single cell RNA sequencing studies have identified subsets of stromal reticular cells that express these SLRPs within LNs. An understanding of the cellular source, molecular interactions and signaling consequences will lead to a fundamental understanding of how SLRPs modulate immune responses, and to therapeutic tools based on these SLRPs in the future.

The Landscape of Small Leucine-Rich Proteoglycan Impact on Cancer Pathogenesis with a Focus on Biglycan and Lumican

Cancer development is a multifactorial procedure that involves changes in the cell microenvironment and specific modulations in cell functions. A tumor microenvironment contains tumor cells, non-malignant cells, blood vessels, cells of the immune system, stromal cells, and the extracellular matrix (ECM). The small leucine-rich proteoglycans (SLRPs) are a family of nineteen proteoglycans, which are ubiquitously expressed among mammalian tissues and especially abundant in the ECM. SLRPs are divided into five canonical classes (classes I-III, containing fourteen members) and non-canonical classes (classes IV-V, including five members) based on their amino-acid structural sequence, chromosomal organization, and functional properties. Variations in both the protein core structure and glycosylation status lead to SLRP-specific interactions with cell membrane receptors, cytokines, growth factors, and structural ECM molecules. SLRPs have been implicated in the regulation of cancer growth, motility, and invasion, as well as in cancer-associated inflammation and autophagy, highlighting their crucial role in the processes of carcinogenesis. Except for the class I SLRP decorin, to which an anti-tumorigenic role has been attributed, other SLPRs' roles have not been fully clarified. This review will focus on the functions of the class I and II SLRP members biglycan and lumican, which are correlated to various aspects of cancer development.

Lumican suppresses cell proliferation and aids Fas-Fas ligand mediated apoptosis: implications in the cornea

Lumican, an extracellular matrix (ECM) keratan sulfate proteoglycan, binds fibrillar collagen and limits collagen fibril diameter in the cornea, skin and tendon. Lumican-deficient mice (Lum(-/-)) develop abnormally thick collagen fibrils, translucent corneas and fragilities of the skin and the tendon. In addition to modulating interstitial ECM structure, here we hypothesized that lumican regulates proliferation and apoptosis of cells residing in the interstitium. Corneal and embryonic fibroblasts from the Lum(-/-) mouse show increased growth in culture. Lum(-/-) mouse embryonic fibroblasts (MEF), compared to their wild type counterparts, display increased rates of proliferation and decreased apoptosis. Ectopic expression of lumican in Lum(-/-) MEF or exogenous recombinant lumican in the culture medium reduces proliferation to rates seen in the Lum(+/+) MEF. We further investigated the implications of lumican's proliferation and apoptosis regulatory role in the cornea where lumican is a major component of the stromal matrix. Stromal keratocytes undergo proliferation and apoptosis during corneal maturation and in the healing of injured cornea. The Lum(-/-) mouse shows increased proliferation and decreased apoptosis of stromal keratocytes during postnatal corneal maturation at the 10-day age. Apoptosis is also significantly down regulated in Lum(-/-) vis-à-vis Lum(+/+) mice during stromal wound healing in the adult 6-week age. Lumican appears to regulate these cellular functions by modulating specific cell growth and apoptosis mediators. Thus, Lum(-/-) MEF have decreased p21(WAF1/CIP1), a universal inhibitor of cyclin-dependent kinases and a consequent increase in cyclins A, D1 and E. Furthermore, the tumor suppressor p53, an upstream regulator of p21 is down regulated in the MEF and the cornea of Lum(-/-) mice. The evidence suggests regulation of p21 by lumican in a p53-dependent way. The MEF and the cornea of Lum(-/-) mice also show a dramatic decrease in Fas (CD95). The Lum(-/-) MEF fail to induce Fas upon treatment with Fas ligand. Fas-Fas ligand interaction is an initiating event in apoptosis and its disruption in lumican-deficiency may partly explain the observed decrease in apoptosis. Fas-Fas ligand mediated apoptosis is critical for maintaining the immune privileged status of the cornea, which implies a new and exciting role for lumican in the cornea.

Purification and characterization of N-acetylglucosaminyl sulfotransferase from chick corneas

N-Acetylglucosaminyl(GlcNAc) sulfotransferase, which transfers sulfate from adenosine 3'-phosphate 5'-phosphosulfate to GlcNAc at the nonreducing end of oligosaccharides, was purified 887-fold with a 8.4% yield from 2-day-old chick corneas by chromatography on CM-Sepharose, WGA-agarose, GlcNAc-agarose, and 3',5'-ADP-agarose columns. The purified enzyme has an optimum pH of 6.0 (Mes buffer) and specifically transfers a sulfate to GlcNAc at the nonreducing end but not to internal GlcNAc. The enzyme was stimulated by protamine and Mn(2+). SDS-polyacrylamide gel electrophoresis of the purified enzyme still showed two main bands (66 and 55 kDa) with some minor bands. It appears that this enzyme competes with beta-galactosyltransferase in binding to the nonreducing GlcNAc residue on KS synthesis; this suggests that the sulfation of the GlcNAc residue is coupled with the elongation of the KS chain.

Comparative analysis of proteoglycans synthesized by chick corneal stromal cells in cell culture and organ culture

Proteoglycans (PGs) synthesized by chick corneal stromal cells in cell culture and organ culture were metabolically radiolabelled with [35S]sulfate (for glycosaminoglycans) and [3H]leucine (for core proteins). Media, cell extracts and organ extracts from cultures were chromatographed on DEAE-Sephacel columns and separated into three fractions: the pass-through fraction (Fraction 1: nonsulfated PGs, hardly sulfated PGs, or glycoproteins with some oligosaccharides), the fraction eluted with a low salt concentration (Fraction 2: undersulfated PGs), and the fraction eluted with a high salt concentration (Fraction 3: highly sulfated PGs). The PG compositions of each fraction of cell culture and organ culture were then compared. While the proportions of highly sulfated KSPG in Fractions 3 of medium and cell extract of cell culture were both very low compared with those of medium and organ extract of organ culture, respectively, the proportions of highly sulfated CS/DS PG in Fractions 3 of those of cell culture were higher than those of organ culture. On the other hand, the proportions in the 35S activities of nonsulfated or undersulfated KSPG in Fractions 1 and 2 of medium and cell extract of cell culture were comparable to those of organ culture. Furthermore, the proportions of core proteins of undersulfated KSPG in Fractions 2 were higher in cell culture than in organ culture. These results show that, when the cells are cell-cultured, the degree of sulfation of KS chains decreases markedly, but the syntheses of the glycosaminoglycan backbone and core protein are maintained.

Role of lumican in the corneal epithelium during wound healing

Lumican regulates collagenous matrix assembly as a keratan sulfate proteoglycan in the cornea and is also present in the connective tissues of other organs and embryonic corneal stroma as a glycoprotein. In normal unwounded cornea, lumican is expressed by stromal keratocytes. Our data show that injured mouse corneal epithelium ectopically and transiently expresses lumican during the early phase of wound healing, suggesting a potential lumican functionality unrelated to regulation of collagen fibrillogenesis, e. g. modulation of epithelial cell adhesion or migration. An anti-lumican antibody was found to retard corneal epithelial wound healing in cultured mouse eyes. Healing of a corneal epithelial injury in Lum(-/-) mice was significantly delayed compared with Lum(+/-) mice. These observations indicate that lumican expressed in injured epithelium may modulate cell behavior such as adhesion or migration, thus contributing to corneal epithelial wound healing.

Characterization of proteoglycans synthesized by cultured corneal fibroblasts in response to transforming growth factor beta and fetal calf serum

A culture system was developed to analyze the relationship between proteoglycans and growth factors during corneal injury. Specifically, the effects of transforming growth factor beta-1 (TGF-beta1) and fetal calf serum on proteoglycan synthesis in corneal fibroblasts were examined. Glycosaminoglycan synthesis and sulfation were determined using selective polysaccharidases. Proteoglycan core proteins were analyzed using gel electrophoresis and Western blotting. Cells cultured in 10% dialyzed fetal calf serum exhibited decreased synthesis of more highly sulfated chondroitin sulfate and heparan sulfate compared with cells cultured in 1% dialyzed fetal calf serum. The amount and sulfation of the glycosaminoglycans was not significantly influenced by TGF-beta1. The major proteoglycan species secreted into the media were decorin and perlecan. Decorin was glycanated with chondroitin sulfate. Perlecan was linked to either chondroitin sulfate, heparan sulfate, or both chondroitin sulfate and heparan sulfate. Decorin synthesis was reduced by either TGF-beta1 or serum. At early time points, both TGF-beta1 and serum induced substantial increases in perlecan bearing chondroitin sulfate and/or heparan sulfate chains. In contrast, after extended periods in culture, the amount of perlecan bearing heparan sulfate chains was unaffected by TGF-beta1 and decreased by serum. The levels of perlecan bearing chondroitin sulfate chains were elevated with TGF-beta1 treatment and were decreased with serum. Because both decorin and perlecan bind growth factors and are proposed to modulate their activity, changes in the expression of either of these proteoglycans could substantially affect the cellular response to injury.

Glycosyltransferase and sulfotransferase activities in chick corneal stromal cells before and after in vitro culture

The expression type of proteoglycan in corneal stromal cells is known to change markedly when the cells are cultured in vitro. To determine which enzyme is primarily responsible for this change in chick corneal stromal cells, the activities of various glycosyltransferases and sulfotransferases were determined before and after in vitro culture of the cells. The activities of N-acetylglucosaminyltransferase, galactosyltransferase, and sulfotransferase, which are involved in keratan sulfate synthesis, were assayed using pyridylaminated N-acetyllactosamine-containing oligosaccharides as acceptor substrate; the activities of N-acetylgalactosaminyltransferase, glucuronyltransferase, and sulfotransferase, which are involved in chondroitin sulfate synthesis, were assayed using pyridylaminated chondrooligosaccharides as acceptor substrate. Of these enzymes, the sulfotransferase activity toward degalactosylated, pyridylaminated lacto-N-neotetraose and N-acetyllactosamine dimer (probably GlcNAc-6-sulfotransferase) decreased markedly after in vitro culture, whereas the galactosyltransferase activity increased. The chondroitin sulfate-sulfotransferase activities toward pyridylaminated chondrooligosaccharides hardly changed after in vitro culture. The marked decrease in the activity of the keratan sulfate-sulfotransferase corresponds to the marked decrease in keratan sulfate biosynthesis when the cells are cultured in vitro. These findings suggest that keratan sulfate-sulfotransferase (GlcNAc-6-sulfotransferase) is a key enzyme in keratan sulfate biosynthesis and that its decrease is primarily responsible for the marked decrease in keratan sulfate synthesis after in vitro culture.

Lumican regulates collagen fibril assembly: skin fragility and corneal opacity in the absence of lumican

Lumican, a prototypic leucine-rich proteoglycan with keratan sulfate side chains, is a major component of the cornea, dermal, and muscle connective tissues. Mice homozygous for a null mutation in lumican display skin laxity and fragility resembling certain types of Ehlers-Danlos syndrome. In addition, the mutant mice develop bilateral corneal opacification. The underlying connective tissue defect in the homozygous mutants is deregulated growth of collagen fibrils with a significant proportion of abnormally thick collagen fibrils in the skin and cornea as indicated by transmission electron microscopy. A highly organized and regularly spaced collagen fibril matrix typical of the normal cornea is also missing in these mutant mice. This study establishes a crucial role for lumican in the regulation of collagen assembly into fibrils in various connective tissues. Most importantly, these results provide a definitive link between a necessity for lumican in the development of a highly organized collagenous matrix and corneal transparency.

Gene structure of chick lumican and identification of the first exon

Three overlapping genomic clones to chick lumican were isolated and then characterized using restriction enzyme analyses, Southern blot analyses with cDNA probes, and by DNA sequencing. The results showed chick lumican gene to consist of 3 exons with a 2.9-kb first intron and a 4.2-kb second intron. Transcription initiation sites, identified by S1 nuclease experiments using genomic fragments containing exon 1 and by primer extension analysis of RNA, indicated the first exon to be 303 b. Two TATA sequences were 31 and 49 bases upstream of the first exon. The first exon contained all 5' untranslated sequence. The second exon was 896 b and contains 20 b of untranslated sequence, and codes for the start methionine to the end of the 10th leucine rich repeat. The third exon is 880 b and codes for the remainder of the core protein, and 724 b of untranslated 3' sequence. A 1-kb genomic fragment containing a portion of exon 1 and upstream sequence in a luciferase reporter sector showed specific promotor activity in the forward, but not the reverse direction when transfected into corneal fibroblasts. These results show the chick lumican gene to consist of three exons, and that regulatory elements are present within 1 kb upstream of the first exon.

Synthesis of corneal keratan sulfate proteoglycans by bovine keratocytes in vitro

Keratan sulfate proteoglycans (KSPGs) are the major proteoglycans of the cornea and are secreted by keratocytes in the corneal stroma. Previous studies have been able to show only transient secretion of KSPG in cell culture. In this study, cultures of bovine keratocytes were found to secrete the three previously characterized KSPG proteins into culture medium. Reactivity with monoclonal antibody I22 demonstrated substitution of these proteins with keratan sulfate chains. KSPG constituted 15% of the proteoglycan metabolically labeled with [35S]sulfate in keratocyte culture medium. This labeled KSPG contained keratan sulfate chains of 4700 Da compared to 21,000 Da for bovine corneal keratan sulfate. Labeled keratan sulfate from cultures contained nonsulfated, monosulfated, and disulfated disaccharides that were released by digestion with endo-beta-galactosidase or keratanase II. Nonsulfated disaccharides were relatively more abundant in keratan sulfate from culture than in corneal keratan sulfate. These results show that cultured bovine keratocytes maintain the ability to express all three of the known KSPG proteins, modified with keratan sulfate chains and sulfated on both N-acetylglucosamine and galactose moieties. KSPG made in vitro differs from that found in vivo in the length and sulfation of its keratan sulfate chains. The availability of cell cultures secreting corneal keratan sulfate proteoglycans provides an opportunity to examine biosynthesis and control of this important class of molecules.

Characterization of collagen fibril segments from chicken embryo cornea, dermis and tendon

The cornea, dermis and tendon have extracellular matrix architectures with differences in fibril diameter, packing and organization. An early step in fibril assembly is the formation of a striated fibril of discrete length (segment). Fibril segments were isolated from developing chicken cornea, dermis and tendon by physical disruption and the structure characterized. In all three tissues, intact but relatively short fibril lengths were isolated. These segments were asymmetric, having long (alpha) and short (beta) tapered ends. They were also centrosymmetric with respect to molecular packing. Segments isolated from 12- to 16-day corneas, dermis and tendons had identical structures, but their lengths and diameters were distinct. We propose that the increase in length is, at least in part, the result of lateral associations of adjacent segments. In the developing tendon, there is a rapid increase in length and diameter between day 16 and 17, while in the dermis the increase is more linear with respect to time. In the cornea, the fibril segments grow longer, but their diameters remain constant. Disruption of corneas in phosphate-buffered saline yielded larger diameter segments than seen in situ, while tendon or dermis maintained tissue-specific diameters. When corneas were disrupted in buffers that stabilized the water layer associated with the collagen molecules or containing the corneal proteoglycans, then tissue-specific diameters were maintained. These data suggest differences in the stabilization of segments during growth in tissues where diameter increases versus those where diameter remains constant, and this may be related to collagen-proteoglycan interactions.

A conformational study of corneal dermatan sulfate proteoglycan using fluorescence spectroscopy

DSPG is a major proteoglycan of the corneal stroma and is thought to be important for the transparency of the tissue. We have studied its conformation by exploring the microenvironment and dynamics of its lone Tryptophan (Trp) residue using steady state and time resolved fluorescence spectroscopy. DSPG exhibits a doublet Trp fluorescence emission. Such a doublet emission has been observed earlier in the copper protein azurin and in avian lens delta-crystallin. Unlike the above cases where the doublet emission is thought to arise due to vibronic structure or the location of Trp at the interface of interacting subunits, fluorescence quenching, denaturation studies and ANS binding with DSPG indicate the location of Trp at two different environments. Such a situation could arise from the differential glycosylation of the core protein or due to duplexation and aggregation of the glycosaminoglycan chains.

The human lumican gene. Organization, chromosomal location, and expression in articular cartilage

A human lumican cDNA sequence was derived by polymerase chain reaction techniques from RNA obtained from intestine, placenta, and articular cartilage. A contiguous sequence of 1729 bases was obtained corresponding to an observed message size of 1.8 kilobases (kb). The cDNA sequence consists of an 80-base pair (bp) 5'-untranslated region, a 1014-bp coding sequence, and a 618-bp 3'-untranslated region terminating in a 17-bp poly(A) tail. The deduced lumican protein sequence has 338 amino acids, including a putative 18-residue signal peptide. The human lumican gene was shown to be spread over about 7.5 kb of genomic DNA and to be located on chromosome 12q22. The gene consists of 3 exons separated by introns of 2.2 and 3.5 kb. The shorter 5'-intron resides 21 bases prior to the translation initiation codon, and the 3'-intron resides 152 bases prior to the translation termination codon. The lumican message is expressed at high levels in adult articular chondrocytes but at low levels in the young juvenile. This age-related trend in message level is not, however, common to all tissues in which the lumican gene is expressed. Lumican is present in the extracellular matrix of human articular cartilage at all ages, although its abundance is far greater in the adult. In the adult cartilage lumican exists predominantly in a glycoprotein form lacking keratan sulfate, whereas the juvenile form of the molecule is a proteoglycan.

Alteration by transforming growth factor-beta 1 of asparagine-linked sugar chains in glucose transporter protein in Swiss 3T3 cells

GLUT1 protein in Swiss 3T3 cells is a 55-kDa glycoprotein with an N-linked oligosaccharide chain. We previously showed that the 65-kDa GLUT1 protein with modulated glycosylation was induced by transforming growth factor-beta 1 (TGF-beta 1) in Swiss 3T3 cells. To further investigate the altered structures of these sugar chains, the membrane glycoproteins solubilized with Triton X-100 were fractionated by lectin-affinity chromatography. The 55-kDa GLUT1 in control and TGF-beta 1-treated cells showed partial binding to Datura stramonium agglutinin (DSA), whereas the 65-kDa GLUT1 exclusively bound to DSA- and wheat germ agglutinin (WGA)-agarose. The 65-kDa GLUT1 in TGF-beta 1-treated cells was sensitive to endo-beta-galactosidase, which cleaves unsubstituted polylactosamine chains. While the 55-kDa GLUT1 in control 3T3 cells was similarly digested by endo-beta-galactosidase, that in TGF-beta 1-treated cells was resistant to this enzyme. These results suggest that the N-linked oligosaccharides of GLUT1 in Swiss 3T3 cells were altered by TGF-beta 1 to forms with more branched and/or repeated polylactosamines as well as with some substitution in the polylactosamines, resulting in a larger GLUT1 molecule. These GLUT1 proteins were exclusively located at the plasma membrane and served as a glucose transporter. However, the affinity to 2-deoxyglucose was significantly increased by TGF-beta 1, associated with the altered glycosylation of GLUT1 protein.

cDNA clone to chick corneal chondroitin/dermatan sulfate proteoglycan reveals identity to decorin

A 1.6-kb cDNA clone was isolated by screening a library prepared from chick corneal mRNA with a cDNA clone to bovine decorin. The cDNA contained an open reading frame coding for a M(r) 39,683 protein. A 19-amino-acid match with sequence from the N-terminus of core protein from the corneal chondroitin/dermatan sulfate proteoglycan confirmed the clone as a corneal proteoglycan and the homology with human and bovine decorin confirmed its identity as decorin. Structural features of the deduced sequence include a 16-amino-acid signal peptide, a 14-amino-acid propeptide, cysteine residues at the N- and C-terminal regions, and a central leucine-rich region (comprising 63% of the protein) containing nine repeats of the sequence LXXLXLXXNXL/I. Chick decorin contains three variations of this sequence that are tandemly linked to form a unit and three units tandemly linked to form the leucine-rich region. The presence of beta bend amino acids flanking the units may serve to delineate the units as structural elements of the leucine-rich region. Sequence homology within the repeats and the spacing of the repeats suggest that this region arose by duplication. Chick decorin primarily differs from mammalian decorins in the 19-amino-acid sequence that starts the N-terminus of the core protein. Within this region, the serine that serves as a potential acceptor for the chondroitin/dermatan sulfate side chain is preceded by a glycine instead of being followed by a glycine as it is in the mammalian decorins and all other mammalian proteoglycans.